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United States Patent |
5,161,153
|
Westmore
|
*
November 3, 1992
|
Synchronous network
Abstract
A broadcast facility is provided in a synchronous network comprising a
number of nodes (b 20l ) coupled via a star coupler (b 21l ). Data is
transmitted via the network in a sequence of frames. Each node with a
public write to send transmits it to all system nodes (including itself)
in the next available frame. Each node has its own identifying
characteristic, e.g. a unique assigned frequency. These identifying
characteristics determine the notional order in which data packets within
a frame are received thus providing each frame with a timestamp.
Inventors:
|
Westmore; Richard J. (Willingham, GB3)
|
Assignee:
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STC PLC (London, GB2)
|
[*] Notice: |
The portion of the term of this patent subsequent to September 18, 2007
has been disclaimed. |
Appl. No.:
|
594388 |
Filed:
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October 5, 1990 |
Current U.S. Class: |
370/407; 370/425; 370/503 |
Intern'l Class: |
H04L 012/56 |
Field of Search: |
370/3,94.1,94.3,4
379/221
|
References Cited
U.S. Patent Documents
4347498 | Sep., 1982 | Lee et al. | 370/94.
|
4715027 | Dec., 1987 | Mahapatra et al. | 370/3.
|
4745592 | May., 1988 | Gabriagues | 370/3.
|
4839887 | Jun., 1989 | Yano | 370/94.
|
4957340 | Sep., 1990 | Kirklay | 370/3.
|
4972463 | Nov., 1990 | Danielson et al. | 370/3.
|
Foreign Patent Documents |
88/05233 | Jul., 1988 | WO.
| |
88/09093 | Nov., 1988 | WO.
| |
Other References
Ballart and Ching, "SONET: Now It
Wagner and Kobrinski,
|
Primary Examiner: Olms; Douglas W.
Assistant Examiner: Samuel; T.
Attorney, Agent or Firm: Lee, Mann, Smith, McWilliams, Sweeney & Ohlson
Claims
I claim:
1. A data communications network, comprising a plurality of terminal
stations interconnected by communication means, wherein each terminal
broadcasts messages via the communications means to all the other
terminals including itself, wherein messages are transmitted in a sequence
of frames, each frame comprising a plurality of messages one from each of
said terminals, the messages within a frame being received concurrently
and in parallel, wherein each terminal includes means for rearranging the
received messages into the order of reception of frames and, within each
said frame, into a predetermined order.
2. A data communications network comprising a plurality of terminal
stations interconnected by a communications medium via a common point or
star, wherein each said terminal broadcasts messages to the network in a
frame format via the common star, wherein each terminal has means for
determining its distance in frame time from the common star and for
transmitting information relating to that distance to all the other system
terminus, wherein each said terminal has means for transmitting an
acknowledgement of a message received from another terminal, and wherein
each said terminal has means for determining, from the distance of itself
and the other terminals from the common star, the time of each transmitted
acknowledgement such that all acknowledgements of one message for any one
terminal arrive at that terminal within the same frame.
3. A data communications network comprising a plurality of terminal
stations interconnected by a communications medium via a common star,
wherein each said terminal broadcasts messages via the common star to all
other terminals including itself, wherein messages are transmitted in a
sequence of frames, each frame comprising a plurality of messages one for
each said terminal, the messages within a frame being received
concurrently and in parallel, wherein one terminal provides a frame timing
reference for the system, and wherein each said terminal determines, from
the frame timing reference of the reference terminal, its time delay from
the star and transmits that information to all other system terminals
whereby to provide frame synchronization of the network, and wherein each
said terminal includes means for rearranging the received messages into
the order of reception of frames and, within each frame, into a
predetermined order.
4. A network as claimed in claim 3, wherein each said terminal has means
for determining its distance or delay time from the common point or star
and for transmitting information corresponding to that delay time to all
other terminals of the network.
5. A network as claimed in claim 4, wherein each said frame comprises for
each frequency within the frame, a data packet, node status information
and frame acknowledge information.
6. A network as claimed in claim 5, wherein one said terminal or node
functions as a reference node to which the other system nodes are
synchronized at the star.
Description
This invention relates to networks for the transmission of data between a
number of nodes or terminals.
BACKGROUND OF THE INVENTION
Network systems for the transmission of data between a plurality of nodes
or terminals are of increasing interest in the data processing field. For
example, optical systems utilizing fibre optic transmission with various
network configurations, employing either active or passive couplers and
dividers with both wavelength and time division multiplexing, are being
developed at the present time. Uses include broadband overlay for
subscriber access networks and ultra-high capacity packet switching for
telecommunication or parallel processing computer applications. See for
example A. Oliphant "Progress in the development of a digital optical
routing system for television studio centres", International Broadcasting
Convention IBC 88, Brighton, Sep. 88, IEE Conference Publication No. 293
pp 90-94, D. B. Payne & J. R. Stern "Single mode optical local networks",
Conf. Proc. Globecom '83, Houston, paper 39.5 and E. Authurs et al "A fast
optical cross connect for parallel processing computers" Proc. 13th
European Conference on Optical Communication, Helsinki, Finland, Sep.
1987.
Such systems offer capacities which are orders of magnitude greater than
electronic (time multiplexed) networks, complete flexibility of
interconnect configuration, service transparency and considerable facility
for future upgrades.
In order to make a particular connection between the nodes of such a
network the optical receiver in the receiving node must be tuned into the
same wavelength as the required transmitter. The switching and
reconfiguration of connections in the network can be achieved either by
switching the wavelength of transmission with fixed separate wavelength
receivers at each node or by using fixed separate wavelength transmitters
in each node and switched wavelength receiver.
For high speed reconfiguration of the interconnection pattern such as
required by telecoms or computer packet switching applications it is
necessary to devise a very rapid communication protocol between the nodes
for setting up the required interconnection pattern. This is very much
easier to achieve using wavelength switched transmitters and fixed
wavelength receivers because in this case the network becomes "self
routing" with messages automatically directed by the transmitter to the
correct receiver. A good example of such a network is shortly to be
published by E. Authurs et al. "HIPASS : an optoelectronic hybrid packet
switching system" IEEE Jnl. on selected areas of Communications Dec. 1988.
A disadvantage with this type of network is that it requires wavelength
switched transmitter components which are very difficult to fabricate with
adequate performance. A further disadvantage of conventional networks is
the difficulty of providing a broadcast facility in which one node can
broadcast a message to all the other nodes. This can introduce severe
problems in maintaining the correct time sequence of signals.
The object of the present invention is to minimize or to overcome these
disadvantages.
Reference is directed to our co-pending U.S. application Ser. No.
07/432,574 which relates to a multi-wavelength optical network comprising
a plurality of nodes interconnected via a single common passive optical
coupler wherein all signals transmitted over the network are synchronous
at the coupler, each node receiving signals from all the nodes and each
node including wavelength demultiplexing means.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided a data
communications network, comprising a plurality of terminal stations
interconnected by communication means, wherein each terminal broadcasts
messages via the communications means to all the other terminals including
itself, wherein messages are transmitted in a sequence of frames, each
frame comprising a plurality of messages one from each of said terminals,
the messages within a frame being received concurrently and in parallel,
wherein each terminal includes means for ordering received messages into
the order of reception of frames and, within each said frame, into a
predetermined order.
According to another aspect of the invention there is provided a data
communications network, including a plurality of terminal stations each of
which is assigned a unique identifying transmission characteristic, and a
communications medium interconnecting the terminal stations, wherein data
are transmitted via the network in a sequence of frames each frame
containing data from one or more said terminals, and wherein, within any
one said frame, the effective time order of data signals corresponds to a
predetermined order of transmission characteristics
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will now be described with reference to the
accompanying drawings in which:
FIG. 1 is a general schematic diagram of a synchronous multi-frequency
optical network; FIG. 2 illustrates the transmission of broadcast frames
of information around the network, of FIG. 1;
FIG. 3 shows a typical frame format; and FIG. 4 is a schematic diagram of a
system node.
DESCRIPTION OF PREFERRED EMBODIMENT
In the network of FIG. 1, a number of identical node equipment 20 are
interconnected by optical fibre transmission paths 21 via a common passive
optical star coupler 22. The star provides a reference point through which
all transmissions are routed. The design of the star coupler is such that
there is only a single path from each input to every output. In the
example shown there are sixteen nodes, the node illustrated in more detail
being number 16. It will however be appreciated that the technique is not
restricted to this particular number of nodes. Each node has a single
wavelength optical transmitter 23, the wavelength being different for each
node 20 of the system. Signals from any one node are propagated via the
fibre and star coupler network to all system nodes, including the node
which originates the signals.
Each node also includes a wavelength demultiplexer 24 wherein the signals
from all the nodes are demultiplexed into sixteen channels and applied via
a multichannel receiver array 25 to an output circuit 26. Typically a
wavelength demultiplexer uses a diffraction grating and lens to direct
each separate channel wavelength signal to a separate detector.
One system node. e.g. node 1, functions as a reference node, and all other
nodes are synchronized with this reference node at the star coupler. To
effect this synchronization, each node (except node 1) adjusts its frame
transmission timing so that its returning signal is synchronized with that
arriving from node 1. This provides automatic compensation for the
different propagation delays between each node and the common star
coupler.
When the system is turned on, all nodes synchronize their frames with
frames from the reference node without regard to phase. The reference node
may be determined e.g. by selecting the lowest wavelength present at the
time of initial system configuration or by some other mutually agreed
method which allows each node to determine the reference node
independently. Each terminal or node determines, from the delay between
transmission and receipt of its own message, its own distance in frame
times from the star or reference point of the network. This information is
transmitted by each node to all the other nodes attached to the system.
From this received information and from the determination of its distance
from the star, each node determines the correct delay to apply to each
acknowledgement such that all node acknowledgements for a particular
message arrive at the sender in the same frame. I.e. all acknowledgements
arrive together at the sending node. In an alternative embodiment, each
node may apply a delay sufficient to accommodate a network containing a
node on a maximum radius arm. This approach reduces the complexity of the
start-up procedure but imposes the maximum acknowledgement delay which
could occur in any network on all networks.
Each node with a public write to send transmits it on its own optical
frequency in the data packet of the work frame. No arbitration or priority
resolution is required. The frames are fixed in length and may be short in
comparison to the time taken for a frame to transit the network. Typically
the frames each comprise 64 bytes and each occupy a time period of 0.5 to
1 microsecond. The transmission format is illustrated in FIG. 2, which,
for clarity shows a four terminal network. In FIG. 2 the frames are
numbered to indicate their relative timestamp positions. All nodes receive
all the data packets from all nodes, including their own.
The frame format is illustrated in FIG. 3 of the accompanying drawings. As
can be seen from FIG. 3, each node writes in data at its own node
wavelength =.sub.n. Each frame can carry in parallel, data from all the
system nodes thus providing efficient use of the system bandwidth. The
first column of the frame may contain a timing/supervisory bit.
The received data packets within a frame are treated as having been
received in the same order in which the wavelengths are numbered. This is
equivalent to resolving timestage order using a number issued by a token
circulating the central star in the order of the wavelength numbering,
pausing briefly at each node to issue it with a number before incrementing
itself and proceeding to the next node. Consequently, each node remains in
a fixed time order relative to the other nodes of the network. As the
frames are short in comparison to the transit time, a high degree of
timestage resolution is provided. In an alternative embodiment, each
terminal may prefix its message with a unique code identifying that
terminal for time ordering purposes.
Referring now to FIG. 4, each system node includes a wavelength
demultiplexor 41, e.g. a diffraction grating, having a plurality of
outputs one for each system wavelength. The demultiplexor outputs are
coupled each to a corresponding receiver 42 the output of which is fed to
a series/parallel converter 43.
The outputs of the series/parallel converters 43 are coupled each to a
first-in-first-out (FIFO) store 44. Readout of the FIFO store in the
correct sequence within each frame is controlled via memory access
circuits 45 by a general control circuit 46. The control circuit 46 also
controls transmission of signals from the terminal via a further FIFO
store 44a and a further memory access circuit 45a. The transmitter signals
are then fed via a parallel/series converter 47 to a transmitter 48
whereby the signal is launched into the network. A network control circuit
49 determines synchronization of the terminal with the network. The system
node of FIG. 4 order receivers messages into the order of reception of
frames and, within each frame, order messages according to their
corresponding wavelength.
The latency experienced by a node waiting to receive its own message back
from the centre star, and hence resolve its own time stamp order, is twice
the delay imposed by its own arm of the network. The effect of this is
that nodes with short links to the centre of the star experience less
latency than more distant nodes. This situation is illustrated in FIG. 2
where node 4 experiences the shortest latency and node 1 the longest
latency.
The network provides message acknowledgement at the physical level. The
acknowledgement of receipt of messages in each node is delayed by a number
of frame times sufficient to ensure that the acknowledgment arrives at the
centre star at the same time as the undelayed acknowledgment for the same
frame from the most distant node. This allows all acknowledgements for a
given frame to be received back at the transmitting node simultaneously n
frames later.
Each node also transmits status information to assist with rapid error
detection and flow control. The exact nature of the status information
required has not been determined but may include: not-ready, I am alive,
and no-errors-detected.
The network described above is of particular application in the coupling of
a plurality of computers or data processors having shared data. It is not
however limited to this application and may also, for example, be employed
in the interconnection of a number of telephone exchanges for the
interchange of informing data.
It will be understood that whilst the technique is of particular advantage
for optical networks, it may also be employed in non-optical systems.
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